The transparent films to be used as optical films represented by retardation films which are in use as optical compensation films for liquid-crystal displays and the like are required to have thickness evenness as well as transparency. In optical compensation films, for example, high thickness unevenness results in an increase in unevenness of phase retardation, which is the product of birefringence and thickness, making it impossible to perform even optical compensation throughout the whole display screen. There has hence been a problem that display devices having excellent display quality including viewing angle characteristics cannot be obtained. In addition, the areas of retardation films are increasing in recent years with the size increase in liquid-crystal displays, and a reduction in thickness unevenness is effective also in improving yield. In particular, when a film is produced through a stretching step, there are the cases where partial stretching occurs to give a thin central part and thick edge parts, resulting in an increased difference in thickness between the central part and the edge parts. A more even film has hence been desired.
Since polycarbonate resins are transparent, there is the possibility of industrially utilizing polycarbonate resins as optical films. Polycarbonate resins are generally produced from starting materials induced from petroleum resources. In recent years, there is a fear about depletion of petroleum resources, and it is desired to supply a polycarbonate produced from a starting material obtained from biomass resources such as plants. Furthermore, since there is a fear about climate changes and the like which are brought about by the global warming caused by increases in carbon dioxide emission and by accumulation thereof, there is a need for the development of a polycarbonate which is produced using a plant-derived monomer as a starting material and which is carbon-neutral even when discarded after use. A technique has hitherto been proposed in which isosorbide is used as a plant-derived monomer to obtain a polycarbonate resin through transesterification with diphenyl carbonate (see, for example, patent document 1). However, the polycarbonate obtained is brown and is not satisfactory when used as an optical film. Meanwhile, it has been attempted to mitigate the stiffness of an isosorbide-based homopolycarbonate, without impairing the optical properties thereof, by copolymerizing isosorbide with a linear aliphatic diol (see, for example, patent documents 2 and 3). Furthermore, a technique is known in which heat resistance and mechanical strength are improved by copolymerizing isosorbide and an alicyclic diol.
However, there has been no knowledge about any technique for inhibiting a polycarbonate resin containing a constitutional unit derived from a dihydroxy compound having a specific bonded structure, such as, for example, isosorbide, which is known as a plant-derived monomer, from suffering thickness unevenness when stretched.